This invention relates to a gas sensor element of the type using a change in electric resistance of a transition metal oxide, and more particularly to a gas sensor element having a porously fired mass of titanium dioxide.
In one type of conventional gas sensor the sensitive material is a transition metal oxide which undergoes a change in its electric resistance with the content of a specific component in an environmental gas atmosphere. In practical oxygen sensors of this type titanium dioxide is prevails as the sensitive transition metal oxide, and it is usual to use titanium dioxide (titania) in the form of a microscopically porously fired layer so that the gas subject to measurement may freely permeate into and through the mass of the metal oxide.
A titania type oxygen sensor is of use as an exhaust sensor in an air/fuel ratio feedback control system for an internal combustion engine because the electric resistance of the titania layer in the sensor exposed to the exhaust gas exhibits a sharp change if the air/fuel ratio in the engine combustion chambers changes across the stoichiometric ratio where the excess air factor is 1.0. To enhance the sensitivity and the speed of response of the oxygen sensor it has been proposed to add a small amount of a noble metal such as platinum or rhodium as described in, for example, Japanese patent application primary publication No. 53-11226. To expand the applicability of titania type gas sensors, U.S. Pat. No. 4,416,763 discloses an air/fuel ratio detecting device for use in engine exhaust gases, which is an integrated combination of a titania type oxygen sensor element and an oxygen ion pump using an oxygen ion conductive solid electrolyte such as zirconia. In this device the resistance of the titania layer exhibits a sharp change in response to a change in the air/fuel ratio in the engine across a nonstoichiometric ratio which is above or below the stoichiometric ratio depending on the polarity of a DC current supplied to the oxygen ion pump. Therefore, this device is applicable to both lean-burn engines and rich-burn engines.
In conventional titania type gas sensors it is usual that the functional part including a titania layer and electrode layers is constructed as a laminate formed on a ceramic substrate by a so-called thick-film technique. To enhance the strength of adhesion of the titania layer to the substrate it is favorable to first prepare a green laminate including the substrate in green state and then fire the green laminate at a relatively high temperature such as about 1300.degree.-1400.degree. C. to thereby accomplish simultaneous sintering of the substrate and the overlying layers. In such cases, however, the fired titania layer often has an excessively tightly sintered structure which is low in permeability to gas molecules presumably by reason of the very rapid growth of titania particles under sintering. With such structure of the fired titania layer it is difficult to satisfy the desire for high speed of response of the gas sensor. It is possible to prevent excessive sintering of the titania layer and resultant low permeability of the same layer by first sintering the substrate alone at a sufficiently high temperature such as about 1400.degree. C. or above, then forming a green laminate including a titania layer on the sintered substrate and finally firing the whole assembly at a relatively low temperature such as about 1200.degree. C. or below. In this case, however, it is likely that the strength of adhesion of the fired titania layer to the precedingly sintered substrate is insufficient so that partial peeling will occur at the interface during practical operations of the gas sensor.